It depends on how complicated your configuration is.
When I did this a long time ago I just did it by hand ie. I didn't use a conversion tool I just created a new configuration in a text editor and then copied in the new configuration.
A lot of the commands in a configuration are defaults and you don't necessarily need to replicate them because you will get a new set of defaults when you convert.
If you have a lot of extra configuration eg. QOS then it may be worth using one. It really depends on how well you know what your switch is currently doing.
In terms of IOS image key things are -
1) feature set ie. you need a feature set that supports all the things you are currently doing
2) memory on your supervisor and whether you have enough
3) module support ie. you need to check the release notes for each release because they may not support all your existing modules
Jon