One other suggestion: sort your queue smallest to largest, and display results as they are available. This will give a perception of being more responsive.

If you can get status back from your image converter, maybe display that (50% done or whatever). Or estimate it by timing each conversion (so if your 10k file takes 10 sec, you estimate the 100k file at 100 sec and give a % based on how long its been going).

Other than that, all I can think of is use a different image conversion engine.