Hi @Thanasis,
First of all, please read @WarrenBelz 's answer to this topic:
https://powerusers.microsoft.com/t5/Building-Power-Apps/Get-image-from-Sharepoint-list-or-library-and-show-the-picture/td-p/471561
Please note that you have to use a DocumentLibrary instead of a SharePoint List.
In PowerApps insert 3 galleries with layout set to Title and Image, one for each category : BestCostumeGallery, FunniestCostumeGallery,ScariestCostumeGallery.
Set the following properties of each galleries similar to:
Items -> DocumentLibrary
TemplateFill -> If(ThisItem.IsSelected,SelectedColor,GalleryBackGroundColor) (you can set this color as you like)
Inside Galleries
set Image control -> Image property -> ThisItem.'{Thumbnail}'.Large
set Label control -> Text property -> ThisItem.CostumeName
Outside galleries on the screen, insert a button control and set OnSelect property to:
Patch(ResultList, Defaults(ResultList), {'Best Costume':BestCostumeGallery.Selected.ID, ,'Funniest Cost':FunniestCostumeGallery.Selected.ID,'Scariest Costume':ScariestCostumeGallery.Selected.ID}).
You can insert a label control outside galleries to see the selected item setting the text property to: BestCostumeGallery.Selected.CostumeName ...
Here is my test, of course I'm not a very good designer:
