Hello,
We can use VLOOKUP for this.
In 'Main Table' sheet, in column H (Images), we create a formula using VLOOKUP, which use the column A (Item #) as lookup value, and the range of all rows in sheet 'Lookup Table' as table array (the place to find the value being searched).
If the formula finds the lookup value (from column A in 'Main Table') inside the table array (the sheet 'Lookup Table'), then the formula will return the value from a column in the table array (1st, 2nd, ...), as specified in the formula. In this case, the column that holds the image URLs (column A in 'Lookup Table'). If the formula cannot find the lookup value inside the table array, it will return the text 'N/A'.
I look forward to work on this project for you. Thank you.