Indeed there is, this answer is from this jpeg faq

Subject: 22 How can my program extract image dimensions from a JPEG file?

The header of a JPEG file consists of a series of blocks, called "markers". The image height and width are stored in a marker of type SOFn (Start Of Frame, type N). To find the SOFn you must skip over the preceding markers; you don't have to know what's in the other types of markers, just use their length words to skip over them. The minimum logic needed is perhaps a page of C code. (Some people have recommended just searching for the byte pair representing SOFn, without paying attention to the marker block structure. This is unsafe because a prior marker might contain the SOFn pattern, either by chance or because it contains a JPEG-compressed thumbnail image. If you don't follow the marker structure you will retrieve the thumbnail's size instead of the main image size.) A profusely commented example in C can be found in rdjpgcom.c in the IJG distribution (see part 2, item 15). Perl code can be found in wwwis, from http://www.tardis.ed.ac.uk/~ark/wwwis/.

That link (the tardis one) is dead, or at least it is for me but what looks like the same code is also to be found here

Cheers,
R.