Understanding file sizes, types and resolution   
Acceptable file types and formats

JPEG/JPG (Joint Photographic Experts Group)
JPEG (usually pronounced JAY-pehg) is an ISO/IEC group of experts that develops and maintains standards for a suite of compression algorithms for computer image files.  It is also a term for any graphic image file produced by using a JPEG standard.  JPEG is the best format for photographic images, though it is the largest file size and will take the longest to download.  If size and download speed are important factors, GIF is preferred.

GIF (Graphics Interchange Format)
The GIF (the original and preferred pronunciation is JIF; it stands for Graphics Interchange Format) is one of the two most common file formats for graphic images on the World Wide Web. The other is the JPEG. The GIF uses the 2D raster data type and is encoded in binary. There are two versions of the format, 87a and GIF89a. Version 89a (July, 1989) allows for the possibility of an animated GIF, which is a short sequence of images within a single GIF file. A GIF89a can also be specified for interlaced GIF presentation.  GIF is the smallest format, but isn't the best for accurate color reproduction, as it uses a limited color set.

PNG (Portable Network Graphics)
PNG (pronounced "ping") is the Portable Network Graphics format, a format for storing bitmapped (raster) images on computers. Unofficially its acronym stands for "PNG's Not GIF." PNG was designed to be the successor to the once-popular GIF format, which became decidedly less popular right around New Year's Day 1995 when Unisys and CompuServe suddenly announced that programs implementing GIF would require royalties, because of Unisys' patent on the LZW compression method used in GIF. Since GIF had been showing its age in a number of ways even prior to that, the announcement only catalyzed the development of a new and much-improved replacement format. PNG is the result.  PNG is good for graphics, but just okay for photos.  PNG may not be compatible with all browsers.

PDF (Portable Document Format)
PDF is a file format that has captured all the elements of a printed document as an electronic image that you can view, navigate, print, or forward to someone else. PDF files are created using Adobe Acrobat, Acrobat Capture, or similar products.

should not be uploaded, because they can not be viewed on web pages.