If you are referring to startup screens you can typically make them in older paint programs. I know MacPaint 2.0 is capable of it (there are a few copies of it for sale on eBay right now; it shows up there pretty frequently) and I believe many other programs of that vintage can handle them as well. I also remember seeing a shareware (or freeware) program at one point that converted MacPaint files to startup screen format. (The original MacPaint didn't save in Startup Screen format).
For those new to this type of file, the startup screen format is used for files that can be named "startup screen" and placed in a System 6 (or earlier) system folder. They replace the "Welcome to Macintosh" message with a picture. If you want a message on here, you can use the text tool of your paint program to do exactly that.
Backdrop, a freeware utility, will let you keep that picture on your screen as a desktop pattern under System 6 (search these forums for it; it's been discussed on here before and I think there's a link for it as well). The only limitation is that the file is in MacPaint format, meaning color cannot be used (I believe this restriction applies to startup screen format as well).
EDIT: Just saw Coxy's post. Glad to know there is a modern alternative to MacPaint for creating this file.