Try enabling "snapping". You most likely want to do a "snap to grid". Then you can adjust the grid spacing to "frames". You can also enable "Quantize to Frames". If you enable snapping, events will also snap to where the cursor is located. So let's say, you have a video clip and if you click on the edge of the clip, the cursor will locate to that edge. Now you grab your audio event and drag it, and when the edge of the audio event reaches where the cursor is located, it will snap to the cursor positon which is lined up with your video clip at that location on the timeline.
If you're using background music, you can also try to figure out how many beats per second a song is (get the proper tempo) and then set the default duration of your slides so that it's in sync with the music, possibly taking the length of transitions (if any) into account. This can be done globally, and you can then drag a whole bunch of images onto the timeline at once (with the same global amount of overlap between slides that you want). Of course, it's unlikely that all the songs you want to use have the same tempo, so you'll need to adjust the global clip duration per song.
Get the "Tap Tempo" and Add Markers At Interval scripts either from the Script Collection or VASST (both free).
Unpack them to a/the scripts folder.
Options > Grid Spacing > Ruler Marks.
Load ONLY the song to a track in Vegas.
Right click the ruler and select Measures and Beats.
Launch the Tap Tempo script (Tools > Scripting > Run Script), play the audio, tap the space bar to the beat and get the BPM.
File > Properties > Ruler, add the BPM here.
Run the Add Markers At Interval script, you will now have a marker at the start of each measure.
Right click the ruler and change it to Seconds.
Double-click between two markers, note the Selection Length time in the box to the lower right of the track view. You can double-click > copy it. For a 120 BPM project the selection length will be 2 seconds.
Options > Preferences > Editing tab, New Still Image Length (seconds), enter the Selection Length time here.
Cut-to-overlap conversion - this is the transition durration time. For what you are doing you may want more of a cut between slides rather than long crossfades so very short times here if that's the case.
Now outside Vegas number all your stills sequentially in a separate folder.
In Vegas' Explorer drill to that folder > select all the images > right-click > Add to Media Pool.
In the Media Pool select all the stills and drag to the timeline. Now all the stills will display for the time you set in Preferences. If you have to make an adjustment you can Right-click the first still > Select Events to end. Right-click over the selected stills > Group > Create New. Now you can Control + Drag the right edge of the group and stretch or shrink as needed.
The above assumes you want each image to display for one measure. If you want the images to display for 2 measures for example you would need to do a little math and double the time for the New Still Image Length entry.
There is also a very handy Set Event Length and Crossfades script for making adjustments.
There are also scripts that aren't free that do this all for you. If you post on the Vegas Video forum they will gladly point you to their products.