Saving in Photoshop.

I am not very familar with Photoshop and am using APS 7.0. I have a scanned an image of an artwork which was originally at A4 size. Using APS, I increased the image size to A2 as well as the resolution, the idea being I can print it out at that size. The new resolution is 500 ppi and 421mm x 594mm (A2).

I however cannot seem to successfully save the image as a Jpeg (but can as a GIF) The image saves as a file marked JPRG but APS will not open the saved file at all, "as an unknown or invalid marker type is found".

My printers tell me that they cannot print GIFs.

photoshop 7 right? and the file is jpeg [not jprg?]

what size and resolution did you scan it at - you may pushed it too far in resizing - jpeg is not a very forgiving format due to its compression

for print I would use tif or pdf, but your printer will advise you on this

leadenwinter,
Why not save to TIFF instead?

First make sure that you are in RGB colour space from the top menus (image mode ) and not in some colour space as indexed colour that may be your problem.
When sure you are in the right colour space try scan again do what you want to the image flatten the image before trying to save as a jpeg all should go well now.
Although jpegs are often denounce as a poor image type I see no problem with them at all so long as you save at maximum quality and you have finished editing them they will print just fine but if you have additional work to do then tiff psd as others have stated are the file types to go for.