Dirt inside back lens element

You'll need to send it to a lens repair service. Do not attempt to disassemble the lens yourself; you'll only make the problem worse and may damage the lens beyond repair.

My phone every time I want to dowload any thing from the web it doesn't support

Front and back focus tests are good to run. You need to do it for each prime lens (fixed focal length) you have, and at your favorite focal lengths for wide angle zoom lenses too. That means on this 17-50, you would pick wide 17mm and zoomed in 50mm, and several others between. If you pick 25mm and 35mm to test, you would end up with 4 potentially different adjustments. These would apply only at 17, 25, 35 & 50mm. If you shoot at say 20mm focal length, and assume it's the same as 17 or even 25mm, you may be wrong. Too much back or forward focusing is a problem with the camera body usually - not the lens.

I'd pay attention to the lens. Look for cloudiness on the edges of the glass, scratches and fungus on glass, wetness or oil on the aperture blades, specs of dust inside the lens, etc. All of these things take away value from the lens, as they can adversely affect the image or shorten the useful life of the lens. You check these things by looking directly through the lens (not on the camera) and look at a dark background - then a light colored background to help show dirt and dust inside.

The only other thing is to look at actual images taken with the lens. Enlarge the pictures to 100% or more. Look for fuzzy edges, purple fringe edges, colors. You will see all of these issues the more you zoom in. You have to decide what it acceptable to you and what is not. Expensive lenses capture great color, are very sharp and have minimal purple fringing on hard edges (high contrast areas) in pictures.

Good luck!

Well, that's a great lens and most definitely worth the cleaning/service. I don't know where in the world you are but fungus growing inside a lens is only heard of in high humidity environments and can happen fairly quickly. Are you sure this is a fungus on the inside and not just a clouded smear on the front element? If the lens is having a problem then the camera won't be to far behind. To answer your question more directly "what should I do" my suggestion is to send the lens to a authorized Tamron service/repair center/depot for cleaning. After which get a few of those little moisture absorbing packets and place them in with your photo gear to help reduce the moisture.

Gently tap the lens with a stick and see if the dirt will fall to the side. It may of course migrate back, but as long as it doesn't stick to the lens, it will be out of the way for most pictures when the aperture closes. If it is stuck to the lens, have it professionally cleaned, OR take consulation in the fact that closer dirt is to the "optical center" of a lens, the less effect it has on normal pictures- pictures of bright objects are different though

Only if you get the version with the built-in autofocus motor. The D3000 lacks a mechanical AF coupling so AF lenses which need to be driven by a motor in the camera body will require manual focussing.

Either way, you're far better manually focussing the Quantaray/Tamron 70-300mm: the AF performance is very slow, very noisy and hunts around a lot to achieve focus.

Although the Quantaray and Tamron versions are identical lenses, if you have to buy this low-quality budget model then go for the Tamron: it's absolutely no better lens quality (and usually the same price), but the manufacturer's warranty from Tamron is much better than Ritz and is internationally valid. If you resell the lens then the Tamron will have some value (not much though) and the Quantaray will be near worthless as it's not so widely known, and has a poor reputation amongst those familiar with the brand.

"Cheap" isn't always the same as "good value"...

If by saying the upper right corner of the lens you really mean that there is a dark spot on each image AND the rear display, there may be dirt or dust on the lens itself. Clean the lens with an optical quality cleaner. Do not use paper towels and Windex or the like - as paper towels can scratch the lens.

It is also possible that the dark spot is a "stuck pixel" on the image sensor inside the camera. There is no way to repair a stuck pixel outside of replacing the sensor. Any dark spot (regardless of source) can be edited out by using image editing software. This will allow you to to make the dark spot less noticeable - or practically invisible.

If the dark spot is on the rear display only, it can also be a result of a "stuck pixel" on the display itself. This should not show up an images, but only on the display. There is little that can be done outside of replacing the entire rear screen.

There are other things that can cause these problems, but those listed above are the most common.

I hope this was Very Helpful and good luck!

Contact Tamron USA via their web site or a phone call to get a price quote for the front lens assembly. However, I would strongly suggest that instead you allow Tamron to repair your lens since more than likely the internal roller glides have been compressed due to the impact. This would allow the front lens group to wobble a bit while focusing and zooming, and this would seriously affect the image quality.

I would say you have a bad drum / toner cartridge.

Stuff you receive with vertical lines is probably the sender's dirty scanner.