Creating SIS file FAILED with themes

[ipilsner]

I'm trying to create a .sis file with my theme that i have created but it keeps failing, the message says:

Out of Memory
FAILED while executing bmconv. Check if any of the picture is 16 bit type or having alpha channel.


Can someone explain this to me? is it b/c i have used photoshop to create most of the pictures some of my files are .bmp, .gif, .png

i have checked and made sure all the pictures were 8bit but what does alpha channel mean??