it's important to understand that there are TWO types of computer graphics.... BIT_MAP or Raster with pixels ( Photoshop or GIMP ) Gimp is free.
VECTOR or POST SCRIPT ( Adobe Illustrator or INKSCAPE) Inkscape is FREE!!!!
if yo visit a screen print shops website you will see that they require or prefer Vector.... and they REQUIRE Spot color NOT RGB OR CMYK.. black and white grayscale is spot.
We use a large laser printer or bubble jet printer to print to vellum. Kinkos fed x can print them for you!
PS you canot enlarge low res files from the net... you need original art or a vector recreation.
Heat transfer typically involves colorants, plastics, and energy using equipment.
Silk screening can be done manually thus using less energy. But, whether the silk screening is actually more environmentally friendly probably depends upon the dye or paint medium. Also, how the silk screener handles, cleans, and disposes their waste.
My bias is that tie-dying and batik are greener yet depending upon the process and materials used; also how they are used, handled, and disposed of.