The drop box is split in 2 sections. The first section is for references images and the second one for ready to use decoration files.

Reference Section

• Format of images shall be png or jpg and the size shall not be more than 10MB.
• Files posted shall mainly be scanned images (e.g. sticker sheets) or photos of printed pieces. Ensure that photos are taken perpendicularly to the decoration.
• Materials posted here will be used to manually generate decorations and therefore blurry images, skewed images or images with low resolutions will be deleted.
• Any registered user can decide to work on decorations and can add their name to the list of drawers. Multiple drawers can work on a same file.
• Only file owner and users with decoration rights are allowed to add some notes. It can be helpful to give more indications about what is on the file. e.g. LEGO or bricklink references and names.
• This is recommended to stitch together multiple images of a same part instead of posting multiple files. For example if you take 2 photos for the front and back of a minifig torso, stitch them together instead of posting 2 files.
• Ensure that the filename is short and descriptive as this is used for the item title. e.g. "Sticker 60144.png" for the sticker sheet of set 60144.

Decoration Section

• Only decoration files ready for Mecabricks shall be posted in this section. It could be a decoration map, a bump map or a metalness map.
• Add a note to help us identify the part. Bricklink/LEGO references, links, etc.
• You can write a note to give more indications about the decoration. For example if the decoration shall be used with other maps like a bump map or a metalness map.
• Notes will also be used to let you know what shall be updated if the file is unsuitable.
• Only users with decorations right can process decoration files.
• Custom decorations are not allowed. Stickers and printed parts shall have appeared at least in one official physical set.

For more information about how to draw decorations, see this post.

File Upload

• Only users with a verified account can upload files in the drop box.
• Make sure that you are in the appropriate section before uploading your file (references or decorations).

Decoration Checklist

• File resolution: It shall be similar to the size of the uv map template. If you wish to use a different resolution, explain the reason in the note section.
• Colour values: Colour values shall be the same as the ones stated in the colour chart provided.
• Positioning: Decoration shall be aligned correctly with the uv map template provided.
• Bump Map: If decoration is a sticker, you shall upload a bump map as well. If your decoration re-uses a bump map already in the part database, write down the reference in the note section.
• Metalness map: If background of decorated parts is pearlescent, metal or chrome and/or if decorated areas include metal paint (334, 335, 336, 337), you shall also upload a metal map.
• Edges: Edges of decoration, bump map and metalness map shall be antialiased.

Mecabricks Decoration Gurus

Here is a list of the users with rights on the decorated part library. These guys are kind enough to give a bit of their time to help Mecabricks. Remember that the only one you can abuse if something is wrong is me. Hopefully not too often!
Stas
wlange
UmbraManis
LeGoMaNiAc02
cynaptic
criker
Nisrok
fredus
Sealund
Qwerty7556