You have to make a patch that will replace those images. What you are saying about uploading them into a folder on your phone it can be done only by a Sysgfx patch, that allows the phone to use other images instead of the original ones. Sysgfx is only available on DB2010 phones (ex:W800,K750 etc). For now, on DB2020 we can use that tool that makes the patch. These images are part of one file, and you have to make a patch for that single file, including your altered image.
To extract files from you phone you have to use JDFlasher. It's a plugin for Far Manager. It's like SEFP only better. It can be found around here, somewhere...