When you are not sure how the name is arranged or what part of the name is used for a package, search synaptic with part of the name. In the case of jpeglib, searching jpeg will show a list for you to look through until you find the one you need. If one part of a name doesn't yield the results needed, try another. You can also switch the search from Description and Name to Provided Packages to find packages with what is needed included in them.