I see.
Okay, well what I would do is open Paint.net then have the images you want open on a browser, right click "Copy" then paste into Paint.net.
Alternatively, you can save the image from the webpage, then right click "Open with Paint.Net".
Either way, you open the image in the program and can then start manipulating it.
Before you do any of that though, I'm guessing you're looking to hang your favourite album covers in those funky square frames, right ?
I've got a friend who's done this and they look amazing, but he uses his actual sleeves, leaving the inner sleeve and the record itself in a clear PVC sleeve in with the rest of his (alphabetised) collection. You can't beat the real thing, so that's what I'd do.
Another option - which I've done but have yet to get round to mounting - is to buy another copy of the LP purely for mounting.
I've got five albums ready to go like this, and one of them - ELO's
Out Of The Blue - I deliberately got on blue vinyl so I could frame it with one of these babies :
Having said that, I've just seen these :
Anyway, as usual I'm rambling.
I've actually already done what you want to do but not in 12" frames - I just colour-photocopied six of my favourite jazz covers and mounted them with clip-frames and the quality was fine. I took the LP sleeves to my local Prontaprint or wherever and it cost me about a quid a pop for a laser copy on A3. I then trimmed them using a scalpel and cutting board - I do physical art as well as digital from time to time - and they look bang on. The colours are still gloriously vivid some ten years on ..
What you need to do above all else is to get HIGH RES images of the sleeves you want.
So using Google's search tools, click on "image" then "search tools" then select "size" and "large" and then you'll find the definition you want.
You can then select the appropriate ones, save them to your computer and email them as attachments to send to your printer or stick them on a USB and take them to their place of business.
I've just done all the above with a search for "Sgt Pepper cover" and downloaded a 1200x1200 image.
I then opened it in Paint.net and checked the "Image>Resize" option and found the resolution to be 96 Pixels/Inch (which I'm guessing is the PPI).
I opened another version of the same picture and edited the res from 96 up to 402. I then zoomed in four clicks of the "magnifying glass" zoom tool on both images. There's no difference to my eye, but there may be in terms of digitisation, I don't know.
You'll need to talk to your printer about that, I guess.
I would've thought they'd do that for you though .. especially as you're the customer and they're the expert. It only takes a few moments, after all ..
That's about all I've got on this one, mate - good luck !
(check his prices - it may be cheaper to buy a crappy copy of the original LP on discogs.com with a mint sleeve, believe me !)