You could use a table inside the textbox. By giving that table a fixed width and height, any image you insert into it will automatically be constrained to fit the available space at the correct aspect ratio. Tables inserted into Textboxes can be left, center, or right aligned. For vertical positioning, you can insert a paragraph before the table and adjust its before/after spacing, line spacing and/or point size.

To set the fixed:
• row height, use Table Tools>Layout>Properties>Row>Specify Height>Exactly.
• column width, first uncheck the 'automatically resize to fit contents' option under Table Tools>Layout>Properties>Table>Options. Then set the preferred column width under Table Tools>Layout>Properties>Columns.
You might also want to set the cell margins to 0 all round. It's also a good idea to format the cell paragraph with 0 before/after spacing.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]