Here are different Icons for Projects, IssueTypes and Priority - Partly with Gimp or Acorn files
IssueType Icons has a size: 16x16 pixles
Project Logo's size: 50x50 pixles
Notice that using an alternate size logo works fine; JIRA Scales it - but I have seen PDF renderings (as in exports) that looked pretty bad due to Logo size.
https://confluence.atlassian.com/display/JIRA/Customizing+the+Look+and+Feel