Step 1) Open your Personal Folder and go to the folder that contains the PKHonor Folder. Then continue going to Cache -> Sprites.
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Step 2) Once you are here the Sprites folder, there will be multiple images. Look for the "Background" image and right click --> Edit.
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Step 3) Click on the Paste button and press Paste From
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Step 4) Find an Image that is 765 x 503 px. Or, just make one via Photoshop.
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Step 5) Once you have found the image, click Open and then Save. If you haven't already, close out of your client and then re-open it. This should now bring up the new client! Hoorah!
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That's all it takes! If you want to change other Sprites such as Donator Symbol Colors, or even the background of your bank! You should follow the same steps you did to change your login screen. That's it! Cya guys, thanks for reading mah guide.
EDIT: This is for Windows, since I do not have a mac and do not know how it works, if someone can explain below just tell me and I can edit this. :3