Merging them into one event won't help. In fact, it would make it impossible to do what you want.

However, if you were to create one large, wide image file with all the images pasted side by side then it would be effortless.

To elaborate a bit, an event is a signpost to a single media. Closest you could do easily using separate events is to stack them on separate tracks in a consecutive order on the timeline, and apply track motion to each track.

And don't be afraid to do this. One of the oldest sayings here is, "tracks are cheap."

Steve's method is probably simpler. Put photos 1, 3, 5, 7, etc on one track and photos 2, 4, 6, 8 on another track, offset forward half the duration of the pictures. Use Pan/Crop position to start photo 1 just off the edge on one side and finish just off the edge on the other side. Copy these Pan/Crop settings to all the other photos. Done.

I wouldn't dream of using track motion for this. Way more complicated, requiring lots of keyframes.

If each photo is on its own track, track motion would seem simple as pie, one keyframe per event. That's how I scroll multiple media. Did I miss something above?

(It took me much longer to upload this than it did to make it.)
Pardon Youtube's playback. The video is smooth.

Well, for one thing, Vegas Movie Studio is limited to 10 tracks. So, if you have more than 10 photos then eventually you'll have to wrap back up to previously used tracks and that will involve adding more keyframes to each track (two per event, not sure how you'd get away with only one). You also end up with a lot of vertical scrolling which makes the workflow more difficult and harder to follow.

Pan/Crop with 2 tracks is a lot easier to follow visually as no vertical scrolling is necessary, and it requires zero keyframes.

Thanks guys,
I've now got it cracked!
The 2 track method seems easiest.
Thanks again everyone for all your help - much appreciated.
(by the way, tracks are not cheap in Vegas - Magix has 99 tracks which can be used for any purpose: video, audio, text -anything, now that's cheap!!!, unfortunately it's not quite as flexible as Vegas in other ways but is nevertheless extremely good software).

If I understand the dynamic you want, I did something like this with all the clips on one timeline. I put the images side by side in the order I wanted. Then I applied the PUSH transition between each image. In the push transition dialog window that opens when you drop the transition between images (slides) on the timeline, check the PUSH OFF PREVIOUS IMAGE box at the bottom left of the dialog box, These PUSH transitions are under the TRANSITIONS tab. Start with the PUSH RIGHT transition. You may eventually want to experiment with the other 'directions' and using the coloured borders. If you want a smoothless motion, you will have to experiment with the time-size of one clip/image/slide - or, if you want the image to pause, you can make it longer on the timeline. If your images have a black border on the right and left (because they are not the same ratio as your project), turn off the MAINTAIN ASPECT RATIO for your images. There are different ways to do this switch, try first by right clicking the image > Switches > Maintain Aspect Ratio (unchecked). BUT, maybe I'm missing something with all this discussion of multiple timelines.

The way my Vegas is set (mostly to default settings), my still images default to 5 seconds long when I drop them on the timeline. I shorten them to about 1 second each. This causes the image to pause before it is 'pushed' out by the next image. I found this looks more like a 'real slideshow.'

But, if you want the images to scroll continously with no pausing on the images, this effect can be achieved in a similar way, but it would take longer to explain and I'm not sure you want this to get too complicated.

So, if this 'transition' effect helps - great - if not, I misunderstood the effect you wanted.

JUST A WARNING: If you have a lot of images, and you try to have them all roll across the screen as if on a seamless ribbon, without any pause for each image, you will create a kind of 'seasickness' (I forgot the correct word for it) effect in your audience. It'd be the same as if they were spinning in circles. At the end of the film, the walls would be spinning. I'm not sure this is the effect you want - since it's no longer the 60s. So, think twice about making an extra long rectilinear seamless ribbon effect.