Just use the program to do the work , open logon studio and create a logon then

click on upload and it will make and save the .logon file next to the wall - 

then submit the wall (jpeg) for preview and the .logon file as the file -\no need to zip it 

unless you have to include a permission - then it's ok to zip both and 

submit the zip instead ....  that should fix your problem  

PS there is NO size or res limit - i would make the wall atleaset 

1280x1024 for fullscreen or 1600x1200 ..... first before you make the logon

those sizes look and work just fine for logons ...  

check out my logons and see how I do it ....