There are several different methods...depending on what kind of graphs you're creating (which procedure used):
A.) Device-based graphics (such as from PROC GCHART, GPLOT, etc)
--1) When you are looking at the graph in the GRAPH1 window, select
FILE --> Export As Image
or EDIT --> Edit Current Graph and then do a FILE --> Export from the Edit window;
--2) use ODS HTML with DEVICE=
--3) use FILENAME and GSFNAME and GSFMODE techniques with DEVICE=
B) Template-based graphics (such as from PROC SGPLOT, SGPANEL, etc or the use of ODS GRAPHICS ON/OFF with statistical procedures)
--1) Use the ODS GRAPHICS statement to control the IMAGEFMT= option however, these template-based graphics are already created as external files, so all you need to do is change the IMAGEFMT
Please note that these methods are all SEPARATE methods. That means if you're using ODS from A, you would -not- use method 1 or 3 and if you're using the Export method, you would -not- also use methods 2 or 3. Also you would not mix device-based methods (A) with template-based methods (B).
Searching Tech Support notes and the documentation should provide you plenty of examples and explanations.