Use root explorer and extract all of the theme apk. This will make a folder under /sdcard/extracted. Also extract all from the source apk.
The folders you want to use are /sdcard/extracted/source/res/drawable-hdpi/ and /sdcard/extracted/theme/res/drawable-hdpi
Look at what the files are named in the theme and the source to make sure they match. They do? Good
Use multiselect to pick out the png files you wish to use in the theme. Copy them, and paste into the theme apk in the drawable-hdpi folder. If you are importing from multiple sources you will of course have to do this more than once.
Once you are done adding your files, then use root explorer and zip the files within /sdcard/extracted/theme, there should be 5 IIRC, use multi select and choose zip. Now navigate to /sdcard/speedsoftware/zip and fild the zip file, you will need to rename it themename.apk once done use signapktic and sign the apk so you can install it, use the testkey it works fine. The new apk by default is placed on the root of your SD card.