You can export any image file as PDF one of two ways….
(1) Use Preview:
– Open the image file in Preview (the OS X default app to view image files).
– Click the File menu and choose “Export”.
– Change the “Format” to “PDF” and save wherever you wish.
(2) Use your printer’s software:
– With an image file open in Preview or any other app, press command P (or File > Print).
– Look for a drop-list option for “Save as PDF” or “Print to PDF”.
After you have a collection of PDF files in a folder (such as one named “Portfolio”), right-click (control-click) that folder and choose “Compress ‘Portfolio'”. That will make a file called “Portfolio.zip”. Upload that.
If whoever is expecting this upload wants one PDF file for all of your portfolio, use Pages app (or TextEdit in RichText format or Microsoft Word) to make a document with all your images. Then use method (2) above.