To simply answer your question, no, you cannot insert any kind of graphic file into a footer. There are, however, a couple of possible work-arounds.
1) check to see if your printer provides its own header/footer or watermark capability. If it does it should show up somewhere in the Excel Page Setup menu. These page setup capabilities appear to be Excel or application capabilities, but in reality are part of the printer driver, so you only get the capability if your printer driver provides it. Chances are, it your printer does support something like this, you can give it a graphic file (.gif, .bmp, .jpg, etc.).
2) This will always work, but only for monochrome graphics like logos and things like that: convert your graphic to a TrueType font character. Excel can print headers and footers in any installed font, so once you get it in TrueType format and install it as a font, you can add it to a header or footer. We have done this here at my company, and it works quite effectively. These fonts are entirely scaleable. Converting your logo to TrueType is a little difficult, however, requiring special software and a learning curve. If you decide to go this route, I suggest you farm the job out.
I hope this helps.
Damon