I don't know about Excel on a Mac but this is how I would do this on a PC ...
in an adjacent column type '001 in the row for the first photo number then copy this down with the fill option to 1049, then position the cursor in the column you want to fill and click on the Fill button in the ribbon and select flash fill. Then you can remove the adjacent column detail.
Thank you for your reply. My Fill button doesn't have the flash fill option.
I tried creating a column starting at 001 and ending on 1049 and then merging the cells, but I get a message saying "The selection contains multiple data values. Merging into one cell will keep the upper-left most data only."
If I click OK it removes rather than append the values from the right column into the left column.
Thanks Lady Audio. It is a shame the flash fill did not work for you as it involved no formulae at all. If you wanted to avoid the formula change where you hit photo number 1000 and the need for the leading 0 is no longer, you could do it this way. Type ' 000 in the first row in column B then drag and fill down ... if Mac version is the same as Windows then this will give you a list of text values from 0001 to 1049 preceded with a space so you can then just concatenate column A and B values. [Jamil's formula achieved this using the TEXT function].