STEP 1
create three new folders in your gallery or memory card and open your blue ftp and copy a theme to the 1st folder,open the copied theme,press menu,select all and extract into the 2 nd folder,copy the pictures you also want to use into the 3 rd folder
STEP 2
write down all you have in the second folder as it is written in small and capital letter it very important,rename your picture in the 3rd folder to what the you written down as it is written in capital or small letters exception of the this theme_descriptor.xml e.g assuming the theme contain just 4 contents Space.gif,opener.png, sett1. gif and theme_descriptor.xml and you want to use 3 pictures,rename the pictures to Space.gif,opener.png,sett1. gif and leave the theme_descriptor.xml so in your 3 rd folder you have your renamed picture go back to the 2 nd folder you will see theme_descriptor.xml copy it to the 3 rd folder so in the 3 rd folder you will have your renamed pictures and the copied theme_descriptor.xml
STEP 3
In the 3rd folder where you have your renamed pics press menu and select all,press menu again and click on compress items(nth) press ok and it will start to compress, when it is done exist blue ftp and you will find your theme in the 3rd folder with the other files.