A simple solution for Crystal Reports XI is to pass the image's URL as a parameter to the report, then use it as the picture's graphic location.
- Create a string parameter field, call it 'imageUrl'.
- Add an image (Insert | Picture...) to the report (it's a placeholder)
- Change the image's Graphic Location:
- right click image
- select Format Graphic...
- select Picture tab
- click the conditional-formula button (looks like x+2)
- set the formula's text to '{?imageUrl}' (without the single quotes)
- save the formula and click the OK button
- save the report
Run the report and set the imageUrl's value accordingly.
Alternately, you could use a CASE statement in a formula field for the same effect; simply change the image's conditional-formula field to reference the formula field instead of the parameter field.