Are you trying to replace the WiFi bars with your custom image?
If so, try this. Set up a theme folder (name it whatever you want, but it must end in .theme). Then, inside that theme folder, put another folder titled UImages. Inside that folder, you will need 8 PNG picture files (that are 21 x 21 pixels in size). They must be titled
Black_0_WifiBars.png
Black_1_WifiBars.png
Black_2_WifiBars.png
Black_3_WifiBars.png
Silver_0_WifiBars.png
Silver_1_WifiBars.png
Silver_2_WifiBars.png
Silver_3_WifiBars.png
Put this into the themes folder of your iPad (/var/stash/Themes.../yourtheme.theme/UIImages/Black_0_WifiBars.png [<--- then Black_1_WifiBars.png, etc.]). Remember that everything must be exactly named, to include upper and lowercase lettering.
Then, open Winterboard, select your new theme, respring and you should be all set.
Now, if you put only one image in there, named all those different names, you won't see any changes for WiFi strength. So, either use different images to signify the bar level - or just be happy you got an image in the status bar (and it'll never change to show signal strength).
If replacing the WiFi bar isn't what you mean ... never mind and I don't know how to help. If you need more clarification - let me know...
Good luck.
Marilyn