Find the Blockland Directory(Or Blockland Folder in that matter), Look for the file named "Base".
Click on it.
Look for Client.
Click it.
Go to UI, then drag the Custom Image into the file.
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To make a custom crosshair, make sure its TRANSPARENT and that it is a .png Image.
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Heres a custom crosshair I made, enjoy.
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DO NOT PUT THE loving CROSSHAIR IN THE ADD-ONS FOLDER!