hi JG53, Tuomo's given you the important gist of the process, now it's time for me to chime in w/ some Important Counterman Arcana.
1st, in Winzip, menu to Options > Configuration > View tab > Columns section & check 'Path' ON. OK the dlog, then do an Options > Save Settings Now.
2nd, after opening your (renamed) VASL.zip in WinZip, click on the 'Paths' label at the top-right of the file-list, to sort the list by Path. All the ctr/mkr/dragover imagefiles are stored in a dir named 'images', most of the other dirs are full of stuff that you really don't want to mess about with. The 'images' dir contains ~5500 files, they're organized in further sub-dirs (a 2-letter code for National ID, or MS (1/2" marker), ML (5/8" marker), & DO (draggable overlay). Each National sub-dir contains its Sniper & all its 1/2" ctrs, & also has a 'gun' & 'veh' subdir (these subdirs are named 'guns' & 'vehicles' for the Brits, 'cos they're British), where the 5/8" ctrs live. There's also an 'sh' (shared) Nationality, where Planes & Doziers & Horsies & Landing Craft live. The 'images' root has some files for the Moved/Berserk/Fanatic/Heroic flags, as well as some of the VASL dlog buttons.
3rd, you can highlight & extract the whole shebang, or all the Yank Vehicle ctrs, or all the Snipers, and Extract them - but (*MOLTO IMPORTANTE*) you MUST use the WinZip 'Extract' feature and have its 'Use folder names' option checked ON. So's when WinZip is finished w/ the unzip, it'll re-create the <path>/<filename> feature - and your user-selected extract-to dir will contain an images subdir, and all your selected files will show up in their appropriate dir-tree structure. I can't rilly stress enough about how important it is to preserve this dir-tree in the unzip process - w/o this tree, your modified files won't make it back into VASL.zip in the right place, & you won't see any results from your work!
4th, let's say you've Extracted the Russian .50-cal HMG ctr - and he's living in the proper structure, that is, his path/name is <your modfiles dir>\images\ru\ruHMG50.gif. You can mod/replace this image all you want, replace the image w/ that kewl BFG9000 pic you d/l'd. Thing is, that original ruHMG50.gif imagefile is only 48x48 pixels (5/8" ctrs are 60x60 pixels), and VASSAL don't re-size images on-the-fly, your new modded file is gonna haveta be exactly 48x48 pixels, if you use a different size, your new ctr won't align nicely in its stack, & will ID itself to your oppo, even when ?'d. So you're constrained to imagefile sizes of 48x48 or 60x60 pixels for any ctr-artwork you do. & also, if you're doing a 'replacement' image for an existing ctr, you've got to keep the same-exact-case-sensitive filename. VASL/VASSAL is written in Java, which is case-sensitive. Ughfortunately, WinZip is not. If you mod ruHMG50.gif & rename it as RUhmg50.gif, WinZip will replace that file, but VASL/VASSAL won't be able to read it. If you're modding a imagefile, keep the same-exact image size and filename for your replacement file.
5th, OK, so you're finished modding the files in your \images\ dir - you've replaced all the xxSniper.gif images w/ a (60x60-pixel Nationality-appropriate) avatart. To install these new images to your local copy of VASL, you've got to drag your modded \images\ subdir from an Explorer window & drop it onto your copy of VASL.zip. When the WinZip Add/Replace dlog comes up, *DO NOT* enable the 'Use full path' option.
6th, when done, you'll prob'ly need to rename VASL.zip to VASL.jar - Try it 1st to see. Working w/ the Java-d/l VASL.jar file can be wonky, it has some kinda' CRC-check facility. If your modded VASL.zip file horks on start-up after you've renamed it to VASL.jar, what you need to do is go to
www.vasl.org & d/l the 'hobbyist' version, post here again if you need help getting your mod to work properly.
And note, that any mods you make to your own 'local' copy of VASL will only show up for your own display - your oppo will see the unmodified ctrs, unless you send him your mod & instruct him as to how to use it. If you want to share your mods w/ others, it'll be easier by far to make an Extension. But in any case, if you come up w/ some nift' improved artwork, post some examples here! Who knows, you might find yourself dragooned into the ranks of the vaslmapcabal, heh, that's how we add recruits to our ranks.
ctrman
hth, working w/ WinZip can be a bit wonky, 'specially in its v12.0 version.