Hi fellow developers, I want to add decals to pets similarly to pet simulator. What would be the best mobile app to make the decal? Also does anyone know how to make icons similarly to the one below? Thanks.
I want to make decals like this one’s
Pet Icon
I would think they use Photoshop or Illustrator. From what it looks like they take the png model picture and upload that to the software. From there they create a circular green outline with a size 5 stroke. Finally they create a size 2 stroke black around the png. Of course you could have a white background with the png or make it transparent. Multiple ways to go about that, using a clipping mask, etc. They then used a white gradient going from the bottom up to get the glow effect. Then they just put whatever font they used as a top layer.
Of course they make the size to that of what Roblox sets their game icons and badges to.
Mobile app wise, I would try Canva. Though it has it’s limitations, so does a majority of these types of apps.
They used photo editing software like Illustrator or Photoshop.
From my experience with Photoshop (not very much) you can import models that are .obj or not sure about .fbx.
I wouldn’t use photoshop for illustrations though, I’d use Adobe Illustrator, or equivalent design platforms. They usually offer a lot of tools but some might come with a steep learning curve except if you already have prior experience!
Not a design fanatic though, but I’d like to see your final result. Remember, it takes time. 
I also reckon Gimp, it’s also free and has very common traits with Photoshop.
Photoshop is only on the computer right? Is gimp mobile compatible?
I’m not sure, though I do use it alot. However now that I’m re reading your post, I don’t think my comment was necessary. Sorry if it didn’t help
I’m honestly not sure if you’d be able to find any app mobile wise that would provide you with the kind of quality that you’re striving for. If you are serious about wanting to design I would highly recommend using a PC or Mac.
You’ll get a crop of answers here, but this is my take on creating something of this caliber;
Adjust game lighting to how you want, adjust model to the perspective you want. I personally use a chroma key skybox when shooting models so that its easier to pull the model from the screenshot in Photoshop
Take the highest quality screenshot of the model possible
Open screenshot in Photoshop and isolate the model on a transparent background
Copy+Paste isolated model into a new image file, preferably with a pixel ratio that suits what you are trying to design.
Once the model is on a blank canvas you can design around it as you please. I personally have never found the trade off of using Illustrator to create images for Roblox worth it, mostly because the image will become rasterized either way once it’s uploaded to Roblox. I just try to nail down my pixel ratios best as possible.