[oblio]

I'm pretty sure that even at that time compressed formats were used :-)

JPG was released in 1992, GIF in 1987.

[Fnoord]

These were used, yes. I am pretty sure Windows in most of 90s used BMPs, e.g. for backgrounds and in MS Paint. [1] Also, GIF is no replacement for BMP.

[1] https://www.fileformat.info/format/bmp/egff.htm