Screen and/or window capturing has been covered a fair number of times in this forum. Just do a keyword search here.
Most of the solutions (including my own) tend to save the results as a BMP rather than a JPG, as that is all that the SavePicture knows how to do. However, there are JPG libraries available, at least one of which I've recommended in the past.
It is worth knowing, however, that VB can save JPG files, if you use GDI+ rather than GDI. I've mentioned this before, although without code examples.