Photos are normally compressed and sent in a smaller file size with less data being used. If you want to send photos in a higher resolution, you can follow the steps below.

Go to a chat > tap “+” on lower left corner > “Photo Library” > Pick a photo > tap “Next” > tap icon on top toolbar > Select “High Quality” > Send

For photos sent in high quality, there will be a icon shown at the lower left corner on the photo thumbnail.

Note:
If the original photo is not in high quality, it will only be sent in standard quality.
If you are sending a few photos in one batch, once you have selected HD in one photo, all other photos will be also sent in high quality (if original photos are in high quality).