图像大小优化，减少数据使用

I'm working on a Instagram-like app on iOS, and I'm wondering on how to optimize the file size of each picture so the users will use as least data as possible when fetching all those pictures from my backend. It seems like it will drain data if you download the file just as it is since high resolution pictures are around 1.5MB each. Is there a way to shrink the size of the picture while maintaining the quality of the picture as much as possible? I'm working on a Instagram-like app on iOS, and