Can I customize the installation image prior to install?

Not really.  Since TLXOS is intended to be a product rather than a custom solution, our main strategy is to perform post-install configuration using TMS rather than pre-install customization.

ISFF and RePC installation can be customized to a very limited degree by editing the *.txt files in the VFAT filesystem after you have imaged the USB stick (and before you use it).  This will allow you to set web proxy settings, the TMS server address the device should use, credentials for connecting to a WiFi access point (or Ethernet switch using 802.1X), and the device's default operating mode (HDX/Horizon/RDP etc).

Unfortunately we can't use the same approach for TLXOS RPi and RPi IoT, because in those cases the installer is never actually run - instead, you are deploying a ready-to-boot system directly onto the SD card that the device will use.  Because we don't know in advance what size SD card you will install on, creation of the /config area (where per-device settings are stored) doesn't occur until first boot, so it isn't possible to preload the filesystem with initial settings.

You are welcome to try to modify the install image anyway, but ThinLinX will not provide ongoing support for any such modifications (i.e. you must re-tailor your image for every new version of TLXOS we release).  A better option would be to deploy complex customizations after install using hotfixes, but unfortunately at present all hotfixes much be encrypted by ThinLinX, so you cannot create your own.  This will change in TLXOS 5.0.

The Windows self-extractor executables that ThinLinX provides are just 7-Zip archives containing the install(er) image and a simple freeware tool (Win32DiskImager) used to write it to removeable media.  You don't have to use Win32DiskImager  - you can use 7-Zip, including the Linux version (7z, from the p7zip package), to extract the ".img" file and write it to the USB stick / SD card using whatever imaging tool you wish (Rufus/dd/rawrite/whatever).

In the ISFF and RePC cases, the install image is a concatenation of three main filesystems, the ThinLinX Firmware Maintenance filesystem (/tfm), the boot filesystem (/boot) and the base root filesystem (/actualroot), in that order.  In TLXOS 4.x, the boot filesystem starts at (512-byte) sector  number 49152 (NUC32/NUC64, a.k.a. ISFF) or 131072 (RePC), and the root filesystem starts at sector number 98264 (NUC32/NUC64) or 262104 (RePC).

In the RPi and RPi IoT cases, the image is a complete MSDOS-partitioned disk, with the boot filesystem on partition 1, the TFM filesystem on partition 2, and an LVM physical volume on partition 3 that contains the root filesystem in a logical volume named "sdcard/root".