and
I mean like in the app not the settings of the backgroundImage
To set the background image of your app you use the blocks which have been provided by @TIMAI2 earlier
Let me suggest you to do some tutorials to learn the basics...
A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook App Inventor 2 Book: Create Your Own Android Apps ... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.
There is a free programming course here Course In A Box and the aia files for the projects in the book are here: App Inventor 2 Book: Create Your Own Android Apps
How to do a lot of basic things with App Inventor are described here: How do you...? .
Also do the tutorials Our Tutorials! to learn the basics of App Inventor, then try something and follow the Top 5 Tips: How to learn App Inventor
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.
???
I am looking for a way for the app to show a button that will pop the user at his file and pick a picture as the background
Try the image picker component together with
Taifun
You might find this useful:
(Read all 104 posts if you want the "big picture" for a complicated subject)
I have a question so I have made what I wanted but there is one issue in my mind is that is there another way because I don't want it to go to Google Photos but instead get it from your files
Where exactly are "your files" ?
Like in my folder like where your downloads go
You mean the Download folder on your Android device ?
/storage/emulated/0/Download
Yes exactly like that
Then what is the problem ?
I want it to go in this folder have on my Chromebook device
That is different.
Will every user of your app be working on a Chromebook ?
It could be like
Is the user detected device = Pc or chromebook or phone then do this --------
No because it could be different but that's what I am on and wanted to test it out
an you do that
do what?
With regard to accessing folders external to your App, you can find diagrams explaining what is valid and what is not on this website. Search for grafik, grafik2 and AndroidOS_Media
Suffice to say that Google have made file access far more difficult than before.
To stop cheating swimmers, they filled the pool with concrete.
So Its not possible for app inventor to open your computers file cabinet
If you setup an http server on your computer, then yes, you can access files served.
Can you show me how its done with a screenshot