Hi Nidhi! According to me the app which I have used for adding monograms to the photos is an app named "Text on Photos with Logo, Emoji & Stickers: Logext" on Google Play Store.
The application has amazing features like " Add logo to the pictures, Watermark your photos , different types of stickers like Birthday stickers, love stickers, thank you stickers and many more".
You can also add text to photo, copyright stickers, trademark stickers.
In short, it is a bundle of all stickers and text to decorate the photos. You should try it once.