Is there some way to search for pictures and add them to Crowbox?

Yes! Use the Share Sheet in your Photos App.
Open the Photos App on your device and use it to search for the pictures you’re interested in.
Then touch Select to choose the pictures you want to use in Crowbox.
Now comes the Share Sheet. Touch the Share button and choose Crowbox. (If Crowbox isn’t on your Share Sheet you may have to edit your Share Sheet to add it.)