Free periodical satellite/aerial imagery
There are a wide ranging of publically available satellite images with different spatial and temporal resolutions. Notably Landsat (https://earthexplorer.usgs.gov/) and Sentinel (https://scihub.copernicus.eu/dhus/#/home) come to mind.
If you are interested in analysing remote sensing products at a global or regional scale, Google Earth Engine (https://developers.google.com/earth-engine) may be a useful tool for you. GEE can directly access Landsat and Sentinel products at no cost to the user.
I hope this information is of use to you.