Yes. You can certainly display both portrait and landscape images in a single Juicebox gallery. (Just add all the images you want to display to your gallery. There is no need to treat portrait-style images any different from landscape-style ones.)

A gallery's dimensions do not change depending on the aspect ratio of the image being displayed.
The gallery's dimensions are determined by the galleryWidth and galleryHeight options (set in the gallery's embedding code).
They can be fixed pixel values or percentages (of the gallery's parent container).
As long as the user does not change the shape of the browser window, then the gallery's size will remain constant.

Images within the gallery will then be scaled according to the imageScaleMode option.
A short description of imageScaleMode (listing all its possible values: SCALE_DOWN, SCALE, FILL, STRETCH and NONE) can be found in the Main Image section of the 'Config Options' page.
The default value for imageScaleMode (used in Juicebox-Lite and when the option is not explicitly set in Juicebox-Pro) is SCALE_DOWN where large images (whether portrait or landscape-style) will be scaled down to fit within the gallery's image area as large as possible, respecting their aspect ratios and without cropping. (Small images are not scaled up as this would reduce their visual quality. You can have small images scaled up if you like by changing the imageScaleMode value to SCALE.)

imageScaleMode is a Pro configuration option (supported only by Juicebox-Pro, not Juicebox-Lite) and can be found in JuiceboxBuilder-Pro's 'Customize -> Main Image' section.

Take a look at this full-page demo gallery which features both portrait and landscape-style images.
You'll notice that the gallery size does not change (unless you change the size of your browser window) but the images (no matter what their aspect ratios are) are always displayed in their entirety.
(This example happens to be a full-page gallery but gallery images would be scaled similarly no matter what gallery size you have.)

I hope these notes help.

I'd recommend that you upgrade to v1.5.0 as many bugs have been fixed (and new features added) since v1.4.3.
Please see the Version History page for a list of changes between versions.

Full instructions for downloading the latest version (and for upgrading JuiceboxBuilder-Pro and existing galleries) can be found on the Upgrading Juicebox support page.