The tumblr cg/l community tends to have a lot of adorable resources when it comes to photos and wallpapers for your phone/computer. The community itself is largely based around reblogging photos pertaining to ddlg or cg/l so it is very easy to find all your adorable needs there! It can get a little NSFW, so if you want something a little more safe just type in pastel in the search bar and you will find equally adorable photos
Get the app called B612, it has a lot of really cute filters and stuff! There are also lots of apps that let you put cute stickers on your pictures! I have one called Kawaii Photo