1) Yes, it's possible to do whatever you wish to do. To archive this, you can use an item editor/object editor for 8.0 clients.
2) You can swap out the images for your new ones and all of those id images will swap out onto that image you've swapped it to.
3) No, no one will do all of this for you. Get started and learn if that's your goal, download an item and object editor. Take your item files, dat file, spr file and get to work and see your results.
Either:
Use Photoshop, gimp or your preferred editing software for sprite editing. There's a lot, select your preferred method.
Or you can use AI to generate nice quality images, not always 100% aligned unless you've chosen one of the best ai generators and detailing your desired image(s) well enough for it to generate it.
If you have experience editing images, you can correct the images pixel by pixel.
The reason nothing like this exists is:
1) time
2) Money
3) Experience
4) Will to give it all away after all that hard work
And to correct this, Ai has come far. It's not perfect but it's at the stage where it's well enough to use fully for design. The best ones cost money, but free ones exists that are extremely well and perfect to use for 2d design.