Hello, Vega. I'm not quite sure what you mean, but I'll help you as best as I can. When you say you have 'an Mp3', from the way you used it, I assume you meant an 'Mp3 Player', like a iPod, or a similar device, correct? If so, I believe I have the solution for you.
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There are two ways to do this. The first (and simplest) is to take the picture you want to use as the cover, rename it to 'folder.jpg' and place it in the folder that contains the mp3 files you want it associated with, like so:
...And it's that simple. Your Mp3 player will read the picture as the album art for all .Mp3, .Ogg, .WAV or whatever is in that folder. The bad news, though, is that you can only use one picture per folder, and everything in that folder will be associated with that one image. So unless you feel like sorting your music library, you might wanna take a look at the other option.
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For the other way of tackling this, you can embed the Album Cover Art directly into the .Mp3 file using ID3 tags. As complicated as that sounds, I'll try to make it as simple as possible.
We'll be using a free program to do this called
Mp3tag. You can download it for free from the developer's website
HERE [2.06mb]
Once it's installed, just open up the file you wanna embed the album cover in with the program.
Once opened, after selecting the file in the window, right click the 'Album Art' panel and click 'Add Cover...'
From the dialog, select the picture you want to use as the album cover (Preferably a .jpg image) and hit OK.
The image should show up in the pane now, so go to the toolbar at the top and click 'save' (the floppy disc icon)
After that, two dialogue boxes will pop up and let you know that you're done.
...and that's it! You can do this for as many individual .Mp3 files as you want and it
should always work. Out of my ~2,000 song library on my 8gb Sansa Fuze, I had maybe 30 that didn't take, and there were technical reasons involved that I eventually resolved.
If you have any trouble, just give me a holler. I'll be checking back in later.