Long time user and just now figuring it out.
So to help those who haven't figured it out yet, yes, you can make your own texture, bump, and decal files using the built in image editor.
Funny; it stares you in the face and you have no idea what you're looking at.
Here is my PTC UPGRADE REQUEST:
A: add a button to the RENDER toolbar for "OPEN IMAGE EDITOR"
B: consider making the image editor a stand-alone app
Now, what am I talking about? How does this relate to creating textures, bumps and decals?
Easy, the PTC "Image Editor" can save .tx1, .tx3, and .tx4 files. Just look in the save drop-down for the various formats available.
You can make the bump file for making actual "tactile" textures (.tx1)
You can make the basic "pattern" to assign to a surface (.tx3)
You can assign an alpha channels if you want only part of an image to show up, like lettering or silkscreen, perhaps (.tx4)
The idea is that many of us have grown up with these formats but know little about them. No standard image viewer know how to show them.
Well, this information opens up a whole new world of manipulating geometry with the basic rendering package provided in Pro/E - Creo/pro
Before this upgrade request is implemented, you can access PTC's Image Editor in 2 ways that I found so far:
1. Open the "Appearance Gallery"
2. "Edit Model Appearances"
3. Click "Map"
4. Insert an Image in any of the 3 entries (if not already available)
5. Right click the image and choose "Preview Texture"
Whallah! The Image Editor opens and lets you do whatever you want.
The second method may be easier;
1. In the "Render" tab, open the "Render Setup" dialog.
2. Click "Output"
3. "Render To - New Window" in the drop-down.
4. "Close" the render setup.
5. Click "Render Window" in the ribbon.
And again, Whallah!... the same Image Editor opens up letting you do whatever you need to do with your favorite texture files.
Now do you see why I'd like a direct link to this editor?
Maybe there is not much use for this in today's version of Pro/E - Creo but it does have some nice features for quick edits on the fly. Of particular interest of course is the alpha channel selection which only works in the decals (.tx4) from what I can tell by the warning messages.
And if PTC wanted to be really nice, they would add an "INVERSE" filter selections since this will reverse the "texture" direction 
Edit: I left out the most important point somehow: You can also reverse engineer your old textures into more common formats 
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