The recommended type of graphic file for printing on clothing is vector. This rule does not apply to photos. They must have a high resolution in order to show up well on the fabric. Modern equipment allows you to take photos with a resolution of 12 MP and higher, so there should be no difficulties.
As for vector files, they can be easily enlarged without loss of quality. The most common vector image formats are AI, EPS, PDF and SVG. The files are available for editing in popular vector editors: Adobe Illustrator, Corel Draw, Affinity Designer, Sketch and others.
A bitmapped photo or picture must have a resolution of at least 200 dots per inch. It will then be sharp and detailed on the garment. You can prepare a photo to create your own t-shirt design in raster editors like Adobe Photoshop.
