Where can I find the MTGO art files?: 
Card art (JPG) can be found at \Program Files\Wizards of the Coast\Magic Online\Graphics\Cards\Pics

Older cards have a resolution of 238x191
(A very few older cards have oddball resolutions)
Newer cards have a resolution of 256x188
Tokens vary, but are usually 256x297 or 256x228
Full art/textless cards have a resolution of 256x367

When you change your art files, the changed art will not be visible immediately, as the MTGO client caches images. They will show up the next time you log on, tho!

Why do my planeswalkers not show the art I just copied to the \Pics directory?: 
The art files for Planeswalkers found in the \Pics directory are used for Planeswalker effects on the stack, not for the Planeswalkers themselves!

Planeswalker card art (PNG) is actually a card frame, and can be found at \Program Files\Wizards of the Coast\Magic Online\Graphics\Cards\Frame

Planeswalker card art has a resolution of 256x367
Some things to remember:
1) Card art filenames are not intuitive!: 
Nor are the filenames in any kind of real order, other than vaguely by block. Example: Volt Charge is 134230_typ_reg_sty_010.jpg. AFAIK, there is no complete reference anywhere as to what art file goes to what card. I've been looking for over a year. You're going to have to manually scroll through those 16K image files and look. 

Here is a partial reference (Dissension to M11/NPH) from another post. Kudos to NorthFury for that list.
2) Card art is often re-used from block to block!: 
You won't find the art file for M11 Condemn amongst the other M11 cards, for example.

If you can't find the art file you're looking for with other cards from that set, look in Gatherer for earlier printings, and then look for the card art in with the earliest set that it was printed in. Sometimes cards will have different art with intermidiate printings, and then the same art again with early printings. It can be quite confusing.
3) You will have to do your own cropping and resizing!: 
MTGO will shrink/blow-up "off size" art files, but unless your art file is already at the correct "aspect ratio", it will stretch/shrink/mutilate the visible image on the card. Even at the correct "aspect ratio", the MTGO shrink/blow-up will often produce "artifacts" in your image.