Hi Andre,
It seems like we need to troubleshoot this issue more extensively using ftp. Can you set up ftp access and provide us the access? This is because troubleshooting through the backend(eXtplorer) poses risks and if there are any errors it may bring the site down, thus preventing us from fixing the issue(editing files) when the backend is down.
We appreciate your understanding on this.
Let us know when you have ftp access.

Updated the first post with this information now

Hi Andre,
Do you mean the 'Browse Media' button in the image block(http://take.ms/WYCUe)? If yes, you can click the 'Back' button on top to go back to the menu to choose the file location.
By right, when you click the 'Browse Media' button it brings you to the image list for 'This Post'.

If you previously uploaded an image in another post, you need to choose the 'All Posts' option(http://take.ms/Vn0JO), then select the post where you uploaded the previous image.

I create two sample posts(test please ignore and test image 1). You can create a new post and try to get images from those two posts through the 'All Posts' option.
Let us know how it goes.

P.S: Your post Empieza mi aventura MasterMind actually get the image from My MasterMind journey begins. So, if you want to get that image, you have to select the post where you originally uploaded it(My MasterMind journey begins).