When making a Decal and a Mission Logo, I do the following:
1. Click "New Mod"
2. Enter the Mod Title
3. Double click the newly created mod so it opens
4. Click New Item tab and then New Decal
5. Give it a Name at the top, comment is optional
6. Give it an entity name
7. Give it a Display name
8. Next to "Image" is a blank box, three dots and then an Import button. Click the three dots and a browse window will open and this is where you search for you .PNG file
9. Once located file click open so that the file location appears in the box and then click Import.
10. Wait a few moments then click New Item tab and then New MissionLogo
11. Fill in the details manually for Name, ignore the drop down arrow next to this box.. The Comment is optional and ignore copy from
12. In General give it an Entity Name again
13. In Decal Mod entry, click the drop down and select the decal name you've just created
14. UI File: Click the button that says UI and then navigate to your mods folder, Open the folder with the mod name you're creating, open the UI folder and then select the .tga file in the location and press open.
for me it is defaulted to C:\Users\xxxxxxx\AppData\Roaming\Surviving Mars\Mods
AppData is a hidden folder.
Once you have selected the tga file, you may get a message saying under the box saying "File does not exist" ignore this. Just click back in the box and off again and the error will vanish.
15. Add in your Display Name and any Mod filters such as logo, decal etc
If you're going to upload to Steam do the additional below, if not press save and the above should create the Decal and Mission Logo so in the Logo Select Screen the Logo appears.
Additional Preview Image Steps:
Locate your original png file. Right click and select properties, then click the security tab and copy the object name in that tab. It will be the file location and file name in one.
16. On the left hand side of the edit mod window on the top option (the parent mod) Click in the preview image box
17. Paste in the copied object name
18. Press the upload button.
Doing the above will create a Steam Preview Image and an Ingame Preview Image correctly